The present invention relates to unburnt lead refills (pencil lead) for writing and drawing pencils, particularly for writing and drawing on drawing cloth (transparent foil), consisting of dyestuff, lubricant, filling material and binding agent, and, if required, wax.
These unburnt lead refills as a rule use synthetics, natural or synthetic resins, cellulose derivatives or similar items as binding agent, using suitable solvents or water for solution. Occasionally, hardeners are used. It is also known in the art how to manufacture lead refills without solvent or water, by mixing suitable synthetics or resins with the remaining components and plasticising them by heating. The resultant mass is shaped at a temperature corresponding to the plasticity point of the binding agent used.
In practical use, the braking strength and the elasticity of these lead refills leaves much to be desired. Another disadvantage is that they are either insoluble in water or too easily water-soluble, while the "medium" condition desired for practical use is not attained. The high degree of evaporation of conventional solvents frequently leads to warpage of the usually extruded lead refills. This warpage is the more prevalent for the thinner lead refills. With lead refills without solvents where the constituents are plasticised through heating, there occur, through thermal differences, stresses which also may lead to a deformation of the finished lead refill. Heretofore, these difficulties have not been effectively dealt with.
It is, therefore, an object of the present invention to provide an unburnt lead refill which has the properties of the known synthetic lead refills, but which is distinguished by a maximum of dimensional stability, breaking strength, elasticity and a medium condition with regard to water solubility. The medium condition regarding water solubility is important because the traces of completely water-insoluble lead refills are practically incorrectable, while with too high a water solubility the traces smear very easily.